The Structure and Special Physiology of Insects 33 



and more convex the eyes the wider will be the extent of the visual field, 

 while the smaller and more abundant the facets the sharper and more dis- 

 tinct will be the image. Although no change in focus can be effected, cer- 

 tain accommodation or flexibility of the seeing function is obtained by the 

 movements of the pigment (Figs. 62 and 63) tending to regulate the amount 

 of light admitted into the eye (as shown by Exner), and by a difference in size 

 and pigmental character of the ommatidia (Fig. 64) composing the com- 

 pound eyes of certain insects tending to make part of the eye especially 



FIG, 65. A section through the compound eye, in late pupal stage, of a blow-fly, Calli- 

 phora sarracenicz. In the center is the brain with optic lobe, and on the right-hand 

 margin are the many eye-elements (ommatidia) in longitudinal section. (Photomi- 

 crograph by George O. Mitchell; greatly magnified.) 



adapted for seeing objects in motion or in poor light, and another part for 

 seeing in bright light and for making a sharper image (as shown by Zim- 

 merman for male May-flies, and by myself for certain true flies (see p. 318)). 

 Our careful studies of the structure of the insect eye, and the experimentation 

 which we have been able to carry on, indicate that, at best, the sight of 

 insects cannot be exact or of much range. 



The psychology of insects, that is, their activities and behavior as deter- 

 mined by their reflexes, instincts, and intelligence, is a subject of great inter- 

 est and attractiveness, but obviously one difficult to study exactly. The 



